I. Field of the Invention
The present invention relates generally to the field of water quality enhancement and more particularly to an apparatus and system for permanent extended-detention basins to treat storm water runoff through gravitational settling.
II. Description of the Related Art
Permanent extended-detention basins and other types of detention/retention basins are used in order to impound and temporarily store storm water runoff for a specified period and discharge it through a hydraulic outlet structure to a down stream conveyance system. An extended-detention basin is usually dry during non-rainfall periods. These basins are necessary and designed to provide for one, or all of the following: a) water quality enhancement, b) channel erosion control and, c) downstream flood control. These basins are installed when impervious surfaces are created from new development activities, which include subdivisions, commercial sites and other new developments. Hydraulically speaking, these basins ensure that post-development will not be different from pre-development conditions.
A standard extended-detention basin includes inlet head-walls, which release storm water runoff to the basin from the storm water drainage pipes/system created by the installation of impervious surfaces for new development. Once the storm water runoff travels to the basin, it is detained and released through an outlet control structure at different design rates. The outlet control structure has an inlet side and an outlet side. The inlet side allows the impounded water to enter the outlet structure at different designed rates. These rates provide for water quality enhancement, channel erosion control, and downstream flood control. The outlet side consists of an outlet pipe, which allows the water to drain through the embankment dike. The typical extended-detention basin allows water to pass through the outlet structure up to the 100-year storm frequency (typically, 7.92 inches of rainfall in a 24 hour period). Once a 100-year storm frequency occurs an emergency spillway (indentation channel on and over the top of the embankment dike) is provided to allow the 100-year storm frequency to pass through the basin.
Extended-detention basins are typically designed to treat the first 1.2 inches of rainfall (also known as first flush) of storm water runoff by releasing it over a 24 to 48 hour period. In so doing, the quality of incoming storm water is improved through gravitational settling of the pollutants. To allow for this 24 to 48 hour draw down time water quality inlet (or orifice) must be sized to allow for the water to be detained for this period. The water quality volume is thus needed to calculate the correct size water quality orifice. The water quality volume is determined using a hydrological equation determined by local governmental regulations/standards. This equation factors in all newly impervious surfaces and takes the first 1.2 inches of rain from these surfaces to determine the water quality volume. Once the water quality volume is determined the correct size orifice can be calculated using a hydrological equation determined by local governmental regulations/standards. The water quality orifice is then protected from clogging by a measure determined by the designer/engineer. There have been attempts to filter the debris, including using filter stone surrounding filter fabric that surrounds an elongated perforated pipe which sits on the basin floor and is attached to the outlet structure. This type of filtration system risks becoming clogged and water is unable to penetrate the elongated perforated pipe and thus cannot enter the outlet structure for release. An additional failed method was to drill the correct size water quality orifice directly into the outlet structure and place a ½ round perforated pipe surrounded by gravel in front of it. Often, water would overflow the ½ round pipe thus bypassing the filtration gravel. Therefore, larger debris such as large sediment, branches, leaves, garbage, can clog the water quality orifice of the outlet control structure, requiring maintenance after every rain event. It is therefore necessary to prevent the water quality orifice from becoming clogged so the extended-detention basin function as it was designed and the first 1.2 inches of rain drains in 24–48 hours.
After the first 1.2 inches of rainfall channel erosion control becomes the next factor in storm water management. To protect against channel erosion the 1-year storm frequency (or typically 3.36 inches of rainfall in 24 hours) is detained for a 24-hour period. To achieve the 24-hour draw down time a channel protection inlet (or orifice) is sized to release 3.36 inches of additional rainfall. This channel protection orifice is calculated from the channel protection volume. The channel protection volume is calculated using a hydrological equation determined by local governmental regulations/standards. This equation factors in all the newly impervious surfaces and takes 3.36 inches of storm water runoff from these surfaces to determine the channel protection volume. Once the channel protection volume is determined the correct size orifice can be calculated using a hydrological equation determined by local governmental regulations/standards. The channel protection orifice is protected from clogging by a measure determined by the designer/engineer. With this invention system the channel protection orifice is protected by an elbow piece of pipe that angles down inside the perforated pipe. On the inside of the structure on this pipe is a screwed in end cap where the corrected sized channel protection orifice is drilled at the invert of the cap. A trash rack may also be used to protect this orifice from clogging if the orifice is drilled exactly sized into the outlet structure.
The next concern that extended-detention basins address is flood control. To control flooding the peak flows of the 2-year through 25-year storm frequencies must be controlled so the flows from the developed site do not exceed those from pre-developed conditions at the project boundary. To accomplish this task v-notched weir, different sized orifices, square weirs, or some other method may be incorporated into the outlet structure to accomplish flood control.